The conventional hydrocracking of petroleum fractions is a very important refining process that makes it possible to produce, from excess heavy feedstocks that contain hydrocarbon, fractions that are lighter than gasolines, jet fuels, and light gas-oils that the refiner seeks in order to adapt production to demand. Compared to catalytic cracking, the advantage of catalytic hydrocracking is to provide middle distillates, jet fuels, and gas-oils of very good quality.
The catalysts that are used in conventional hydrocracking are all of the bifunctional type that combine an acid function with a hydrogenating function. The acid function is provided by substrates with large surface areas (generally 150 to 800 m.sup.2 g.sup.-1) that have a surface acidity, such as the halogenated aluminas (chlorinated or fluorinated in particular), combinations of boron and aluminum oxides, amorphous silica-aluminas and zeolites. The hydrogenating function is provided either by one or more metals of group VIII of the periodic table, such as iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum, or by a combination of at least one metal from group VI of the periodic table, such as chromium, molybdenum, and tungsten and at least one metal from group VIII that is preferably not a noble metal.
The balance between the acid function and the hydrogenating function is the main parameter that controls the activity and selectivity of the catalyst. A weak acid function and a strong hydrogenating function provide low-activity catalysts that work at a generally high temperature (greater than or equal to 390.degree. C.) and at a volumetric flow rate at low feed rate (VVH expressed by volume of feedback to be treated per unit of volume of catalyst and per hour is generally less than or equal to 2) but that have good selectivity for middle distillates. Conversely, a strong acid function and a weak hydrogenating function provide catalysts that are very active but have poor selectivity for middle distillates. Furthermore, a weak acid function is less sensitive to deactivation, in particular by nitrogenous compounds, than a strong acid function. The challenge therefore is to select judiciously each of the functions in order to adjust the activity/selectivity pair of the catalyst.
The low-acidity substrates generally consist of amorphous or poorly crystallized oxides. The low-acidity substrates include the family of amorphous silica-aluminas. Some of the catalysts on the hydrocracking market consist of silica-alumina combined with a combination of sulfides of the metals of groups VIB and VIII. These catalysts make it possible to treat feedstocks that have high contents of heteroatomic poisons, sulfur, and nitrogen. These catalysts have very good selectivity for middle distillates; they are very resistant to the strong nitrogen content, and the products that are formed are of good quality. The drawback of these catalytic systems with an amorphous substrate base is their low activity.
The substrates that have strong acidity generally contain a dealuminated zeolite, for example of the dealuminated Y type or USY (Ultra Stable Y zeolite), combined with a binder, for example alumina. Some catalysts on the hydrocracking market consist of dealuminated zeolite Y and alumina, which is combined either with a metal from group VIII or with a combination of sulfides of the metals of groups VIB and VIII. These catalysts are preferably used for treating feedstocks whose contents of heteroatomic poisons, sulfur, and nitrogens are less than 0.01% by weight. These systems are very active, and the products that are formed are of good quality. The drawback to these catalytic systems with a zeolite substrate base is their selectivity for middle distillates, which is not quite as good as that of catalysts with an amorphous substrate base and very high sensitivity to nitrogen content. These catalyst can tolerate only low nitrogen contents in the feedstock, generally less than 100 ppm by weight.